1. Field of the Invention
The present invention relates to a driving device equipped with a rotary inertial body, and an image forming apparatus.
2. Description of the Related Art
Rotary body driving devices that are equipped with a rotary body and a rotary inertial body (flywheel) to maintain a constant rotational velocity of the rotary body are well known. Such rotary body driving devices are widely used as photosensitive drum driving devices in image forming apparatuses such as copiers, facsimile machines, and printers. In an image forming apparatus, image data is written on the photosensitive drum functioning as a rotary body by an optical scanning unit to form a toner image on the photosensitive drum, the toner image is transferred to a recording medium, and the toner image on the recording medium is fixed o obtain the image. It is important to maintain a constant rotational velocity of the photosensitive drum when the image data is being written to the photosensitive drum by the optical scanning unit or when the toner image is being transferred to the recording medium. Any variation in the velocity of the photosensitive drum will cause deterioration in the quality of the toner image or of the image being transferred to the recording medium.
To maintain a constant rotational velocity of the photosensitive drum, it would be advantageous to increase the inertial energy E of the rotary inertial body, which is represented by the equation E=(Jω2)/2 (where J is the inertial moment of the rotary inertial body and ω is the angular velocity of the rotary inertial body). In other words, either the inertial moment J or the angular velocity ω of the rotary inertial body can be increased.
The inertial moment J can be increased by using a heavy and large-diameter rotary inertial body. However, such a rotary inertial body will occupy more space owing to its size, and owing to its weight, necessitates increasing the rigidity of a supporting mechanism for the rotary inertial body, pushing up the cost. The size will also hinder accessing the parts beyond to the rotary inertial body for maintenance purpose.
Driving devices in which angular velocity of the rotary inertial body is increased so as to be greater than the angular velocity of the photosensitive drum are disclosed in Japanese Patent Application Laid-open No. 3013779 and Japanese Patent Application Laid-open No. H10-288915.
FIG. 10 is a drawing of the driving device disclosed in Japanese Patent Application Laid-open No. 3013779.
A driving motor (driving-force source) 105 that drives a photosensitive drum 1 is fixed to a frame 102 of an image forming apparatus. A first small gear 106 is fixed to a first rotary shaft 110 of the driving motor 105, and engages with a first large gear 107. The first large gear 107 along with a second small gear 108 is fixed to the first rotary shaft 110, which is rotatably supported by the frames 102 and 103. The second small gear 108, which engages with a second large gear 109, is fixed to a second rotary shaft 111 (input shaft), which is rotatably supported by the frames 102 and 103. A first shaft joint 112 is fixed to the end of the second rotary shaft 111.
A second shaft joint 113 is fixed to the end of a third rotary shaft 1a, which serves as the rotational center for the photosensitive drum 1. The second shaft joint 113 is fixed to the first shaft joint 112. A first pulley 118 is fixed to the second rotary shaft 111.
A wheel rotary shaft 119 (output shaft) is supported by the frames 102 and 103 of the image forming apparatus. A flywheel 120, which serves as the rotary inertial body and stabilizes the rotational velocity of the photosensitive drum 1, is fixed to the wheel rotary shaft 119. A second pulley 121 is fixed to the wheel rotary shaft 119. The diameter of the second pulley 121 is smaller than that of the first pulley 118. An endless belt 122 is wound around the second pulley 121 and the first pulley 118.
The driving force of the driving motor 105 is transmitted to the second rotary shaft 111 (input shaft) via the gears 106 to 109, which reduce the rotational velocity before it is transmitted to the second rotary shaft 111. As a result, the first pulley 118 fixed to the second rotary shaft 111 (input shaft) rotates, simultaneously rotating the third rotary shaft 1a via the shaft joints 112 and 113, and therefore, the photosensitive drum 1. The driving force of the first pulley 118 is transmitted to the second pulley 121 by the endless belt 122, causing the second pulley 121 as well as the flywheel 120, which is coaxial with the second pulley 121, to rotate. As the radius of the first pulley 118 is larger than that of the second pulley 121, the angular velocity of the flywheel 120 is greater than that of the photosensitive drum.
Thus, by increasing the angular velocity ω of the flywheel 120, which serves as the rotary inertial body, the inertial energy E can be increased without having to increase the inertial moment J. Thus, required inertial energy can be obtained even with a light and small-diameter flywheel 120. As a result, the flywheel 120 can be fitted in a smaller space. Further, the rigidity of the shaft bearing and the frames 102 and 103 that support the wheel rotary shaft 119 need not be increased, thus preventing cost escalation.
FIG. 11 is a drawing of a driving device disclosed in Japanese Patent Application Laid-open No. H10-288915.
The driving device disclosed in the patent document includes a velocity-varying mechanism 130 to increase the angular velocity of the flywheel 120 rather than that of the photosensitive drum 1. The velocity varying mechanism 130 includes a large friction wheel 128 fixed to the second rotary shaft 111 (input shaft) and a small friction wheel 129 fixed to the wheel rotary shaft 119 (output shaft) and engaging with and rotating with the large friction wheel 128.
The driving device disclosed in this patent document also realizes increased angular velocity ω to obtain increased inertial energy E while keeping the radius and weight of the flywheel 120 low.
However, in the velocity-varying mechanism disclosed in the former patent document a large tensile force is imposed on the endless belt 122 so as to prevent the first pulley 118 and the second pulley 121 from slipping. The tensile force causes the second rotary shaft 111 (input shaft) and the wheel rotary shaft 119 (output shaft) to bend towards each other, resulting in wobbling of the flywheel 122 and the photosensitive drum 1. The vibrations generated by the wobbling increases the velocity variation in spite of the flywheel 122. In the velocity-varying mechanism 130 disclosed in the latter patent document, significant pressure is required so that the small friction wheel 129 does not slip off the large friction wheel 128. As a result, the second rotary shaft 111 (input shaft) and the wheel rotary shaft 119 (output shaft bend away from each other, causing the flywheel 120 and the photosensitive drum 1 to wobble.
To avoid slipping, gears, etc., which have better gripping power because of presence of teeth, can be used in the velocity-varying mechanism. However, here again vibrations occur due to backlash or precision of teeth meshing profile.
Further, in the velocity-varying mechanisms disclosed in the two patent documents, the wheel rotary shaft 119 has to be located off a position coaxial with the second rotary shaft 111, increasing the size of the driving device in the radial direction of the rotary shaft.